Liam Denning is a Bloomberg Gadfly columnist covering energy, mining and commodities. He previously was the editor of the Wall Street Journal's "Heard on the Street" column. Before that, he wrote for the Financial Times' Lex column. He has also worked as an investment banker and consultant.

TransCanada, perhaps more than most of its compatriots, has a love-hate relationship with its southern neighbor. To the general public, it will be forever remembered for the tortuous on-off-on-again-off-for-real Keystone XL pipeline affair.

In investing circles, though, it is known chiefly for its defensiveness, with virtually all of its profit coming from long-term contracts or fees based on the cost of service for its pipelines. News that it is in talks to buy another pipeline company fits this posture -- albeit with an important twist.

While unconfirmed, the Wall Street Journal reported on Thursday that the company in question is Columbia Pipeline Group. If that is the case, then one potential motive is to co-opt a growing threat to TransCanada's own position.

Here's a picture of TransCanada's pipeline network in Canada and the U.S. ripped from a recent company presentation:

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Notice the long blue line snaking across left to right on the northern side of the Canadian border toward the Atlantic. That is the Canadian Mainline taking natural gas from western Canada to the country's own eastern provinces and the demand centers of the northeastern U.S.

Which makes this a problem:

Need Any Gas?

Natural gas production has surged in two major shale basins in the northeastern U.S.

Source: Energy Information Administration

The shale boom has upended all sorts of expectations in the global energy market -- only last month, the first shipment of liquefied natural gas from U.S. shale left the Gulf Coast. Canada is no exception. Look at what has happened to flows of gas across the border between the northeastern U.S. and eastern Canada:

Reversing the Flow

The shale boom means consumers in the northeastern U.S. need less imported gas from Canada and surplus gas is also heading north

Source: Energy Information Administration

Note: U.S. natural gas imports and exports through points in Maine, New York, New Hampshire and Vermont.

With natural gas from the Marcellus and Utica shales not only displacing Canadian gas in the northeastern U.S. but also pushing back across the border, TransCanada faces a long-term threat to one of its main pipeline systems. Over time, more and more gas from those shale basins will also migrate westward, competing with volumes shipped by TransCanada to the Midwest.

One way to hedge the threat, of course, is to simply own the pipelines shipping that shale gas, taking advantage of the growth there to offset the pressure it puts on the business elsewhere. And that is where Columbia can potentially help:

As you can see from this map taken from a recent company presentation, Columbia's network is all over the northeastern shales like a curiously linear rash. In buying it, TransCanada would be insuring itself against any more nasty surprises from the south.

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